Magnetotransport in under and optimally hole-doped bulk nanocrystalline [formula omitted] manganites

Abstract

Electrical-transport and magneto-transport in bulk nanocrystalline (BNC) La1-xCaxMnO3+δ (x = 1/8 with either δ = 0.00 or δ = 0.06, x = 3/8 with δ = 0.00) (LCMO) manganite system have been studied treating the bulk crystalline (BC) counterparts as the reference systems. Irrespective of the value of δ in under hole-doped (x = 1/8) LCMO, the metal-insulator (M-I) transition (not observed in the BC counterpart) appears at a temperature TMI≪TC, where TC is the temperature at which the ferromagnetic (FM)–paramagnetic (PM) second-order phase transition occurs. Oxygen off-stoichiometry makes the M-I transition more pronounced and reduces the overall magnitude of resistivity. Reducing the average crystallite size d below 150 nm, lowers both TMI and TC in the BNC samples with x = 1/8 and δ = 0.00 or 0.06 but decreases TMI while increasing TC in the optimally hole-doped x = 3/8 LCMO. The single-orbital double-exchange model, which predicts the coincidence, or otherwise, of the M-I and FM-PM transitions with some success in the BC LCMO, fails to correctly describe the above-mentioned results in BNC LCMO. The resistivity minima at low temperatures (Tmin), completely absent in BC, occur in BNC LCMO, regardless of the value of x,δ,d and the strength of the magnetic field (H). The mechanism for resistivity, ρ(T), changes from the adiabatic small polaron nearest-neighbor hopping (for x = 3/8) or from the Shklovskii-Efros variable range hopping, SE-VRH, (for x = 1/8), prevalent in BC, to the Mott-VRH in the PM insulating state in the BNC counterparts when d falls below 150 nm. At T < Tmin, the contributions due to the Mott-VRH (present only in the samples with x = 1/8 and δ = 0.00 or δ = 0.06) and the enhanced electron-electron Coulomb interaction (in all the BNC LCMO samples) are responsible for the negative temperature coefficient of resistivity (TCR). By contrast, the coherent electron-magnon scattering accounts for the positive TCR observed at T > Tmin. H suppresses the mean hopping energy difference between the hopping sites but increases the electron localization length. The field-induced enhancement in the localization length far outweighs the field-induced drop in the mean hopping energy difference to produce an increase in the mean hopping distance with H.